In a conventional installation block, the flush tank manufactured of plastic is foamed into a self-supporting body made of polyester foam concrete. Supply pipes and connections for water and waste water, wall installation fittings and attachment elements for sanitary devices to be connected are pre-installed accurately and on the operating mechanism side. This installation block is mounted on or in a wall with a brick lining or embedment, or in a light construction wall. In order to be able to attach, if desired, also a water closet with subdouche to this installation block, a fixed copper pipe line, which empties at the top into the flush tank and is connected there to the inlet valve with a separate connecting piece, is foamed into the foam body. The other end of the copper pipe, which empties outwardly, comprises a flange for attaching the connecting pipe to the subdouche of the water closet. Installation of this copper pipe line is time-consuming, since the copper pipe must be bent accurately and foamed precisely into position. The manufacturing costs of the installation block are significantly increased by this copper piping. Access to the interior of the flush tank through the inspection opening is poor, for which reason attachment of the upper end of the copper pipe line to the inlet valve is very time-consuming.